A plastic lens is lighter than an inorganic lens, is not easily broken, and is dyeable. Therefore, plastic lenses have become rapidly widespread as an optical element such as a spectacle lens or a camera lens. Hitherto, various resins for spectacle lenses have been developed and used. Typical examples of the resins include allyl resin obtained from diethylene glycol bisallyl carbonate or diallyl isophthalate, (meth)acryl resin obtained from (meth)acrylate, and thiopolyurethane resin obtained from isocyanate and thiol.
In recent years, urethane resins, which have a refractive index lower than that of thiourethane resins and are formed of isocyanate and an alcohol, have been developed (Patent documents 1 to 5). Although the refractive index of urethane resins is lower than the refractive index of thiourethane resins, the urethane resins are less expensive. Therefore, urethane resins are expected to be used as a lens material. For example, Patent Document 4 describes an urethane resin obtained from 4,4′-methylene-bis(cyclohexylisocyanate) as isocyanate and trimethylolpropane propoxylate and trimethylolpropane as alcohols.
As the isocyanate, there are tolylene diisocyanate and diphenylmethane diisocyanate which have a high refractive index and are available at low cost, and functional lenses or high-refractive index lenses using these polyisocyanate compounds are being studied.
In Patent Document 6, an example of manufacturing urethane resin by using tolylene diisocyanate, which is available at low cost, as the isocyanate is described in Comparative example 5. Examples of commercially available isocyanate include m-xylylene diisocyanate, 2,5-bis(isocyanatomethyl)-bicyclo[2.2.1]heptane, 2,6-bis(isocyanatomethyl)-bicyclo-[2.2.1]heptane, 1,3-bis(isocyanatomethyl)cyclohexane, hexamethylene diisocyanate, isophorone diisocyanate, 4,4′-methylene-bis(cyclohexylisocyanate), and the like. The market price of tolylene diisocyanate is the cheapest among these commercially available products. Therefore, if tolylene diisocyanate is used, cheaper spectacle lenses can be supplied to the market, and it can make a great industrial contribution. However, Comparative example 5, in which the example of manufacturing urethane resin by using tolylene diisocyanate is described, also describes that a cured substance is not obtained in some cases due to molding defect.
Patent Document 7 describes a plastic lens comprised of thiourethane resin that is obtained from a composition containing a polyisocyanate compound having an aromatic ring, a polythiol compound, a benzotriazole-based compound, and phenols. The document describes that a plastic lens having excellent weather resistance is obtained.